The present invention relates to a clarifying apparatus for clarifying insoluble fission products or corrosion products in nuclear fuel reprocessing or clad processing of reactor cooling water.
An insoluble fission product is present in a spent fuel solution (nitric acid solution). The spent fuel solution is clarified to remove the insoluble fission product. The resultant clarified solution is subjected to the next extraction process.
A pulse filter apparatus is frequently used in the conventional clarifying process. In the pulse filter apparatus, the spent fuel solution is filtered through a filter obtained by sintering a stainless steel powder. The insoluble fission product is thus separated from the solution containing nuclear fuel. The sludge of the fission product deposited on the filter is removed from the filter by supplying pulsated air in a direction opposite to the direction of the fuel flow.
The pulse filter apparatus of this type is capable of removing relatively small particles of the insoluble fission product from the spent fuel solution and is capable of decreasing the size of particles left in the clarified solution below a predetermined value, thereby keeping the uniform quality of the solution. In addition, the pulse filter apparatus does not have a movable component, so that the apparatus is free from mechanical problems and has high reliability.
However, the pulse filter apparatus has a disadvantage in that the filter thereof tends to clog. When the sludge is deposited on the filter surface, the filtration rate is decreased. If this occurs, the filter must be replaced with a new one. The filter can be used for two or three months for clarification. However, at the time when the filter is replaced, the clarification process must be stopped for about a week, thus increasing clarification cost. In addition to this disadvantage, maintenance personnel tend to be exposed to radiation. The atmosphere is also subject to radioactive contamination. The filter, as a high level radioactive waste, limits waste disposal. For the above reasons, there arises a strong demand for eliminating replacement of the filter in the pulse filter apparatus.
Meanwhile, the centrifugal clarifier has recently received a great deal of attention. The spent fuel solution is supplied to a cylindrical rotating bowl and is separated by centrifugal separation in the centrifugal clarifier. More particularly, an insoluble fission product is separated from the solution by utilizing the difference between the specific gravities of the insoluble fission product and the nitric acid solution. In a centrifugal clarifier of this type, the flow rate per unit time can be increased. In addition to this advantage, the sludge deposited in the bowl can be easily removed by the remote control. However, in the centrifugal clarifier, it is difficult to decrease below a certain value a minimum size (to be referred to as a critical particle size dp) of insoluble fission product particles which can be processed. The critical particle size dp is given by equation (1) below: EQU dp=(18.multidot..mu..multidot.Q/.DELTA..rho..multidot.g.multidot..SIGMA.).s up.1/2 ( 1)
where
.mu.: the viscosity of spent fuel solution PA1 Q: the flow rate of spent fuel solution PA1 .DELTA..rho.: the difference between the specific gravities of the particles and the nitric acid solution PA1 g: the acceleration of gravity PA1 .SIGMA.: the centrifugal separation/deposition area which is given as follows: EQU .SIGMA.=.pi..multidot.l.multidot..omega..sup.2 .multidot.(r.sub.2.sup.2 -r.sub.1.sup.2)/g.multidot.[1n(r.sub.2 /r.sub.1)] (2) PA1 l: the length of the bowl along its rotating axis PA1 .omega.:the rotational velocity of the bowl PA1 r.sub.1 : the rotational radius between the rotational axis of the bowl and a position at which the solution is extracted PA1 r.sub.2 : the rotational radius between the rotational axis of the bowl and the inner surface thereof.
where
As is apparent from equations (1) and (2), in order to decrease the critical particle size dp, the radius r.sub.2 or the rotational velocity .omega. must be increased. When the radius r.sub.2 or the rotational velocity .omega. is increased, the peripheral speed of the bowl is increased, so that the bowl is subjected to a large centrifugal force. For this reason, the mechanical strength of the bowl must be sufficiently increased, and the wall thickness of the bowl must be increased. However, when the wall thickness is increased, the weight of the bowl is increased. As a result, a high-power bowl drive device is required, and a bearing which withstands a high stress must be used. In addition, the critical particle size dp can be decreased when the length l of the bowl is increased. However, the weight of the bowl is increased. For these reasons, it is impractical to design the centrifugal clarifier to decrease the critical particle size dp. In addition to this disadvantage, the sludge is deposited in the bowl and is gradually increased. As a result, the effective length l of the bowl is gradually decreased, and the critical particle size dp is increased. In this manner, the sludge must be removed from the centrifugal clarifier at a proper time. If the sludge is removed at the late time, the insoluble fission product cannot be removed in the centrifugal clarification process and will flow into the next process. In this manner, neither the conventional pulse filter apparatus nor the conventional centrifugal clarifier will solve the practical problems.